Yes, Gold Stackers buys back unallocated at spot - several dealers advertise lower prices over spot, but it is the spread that ultimately matters. Our spreads for unallocated reflect our business model in that it is being fully backed by retail physical products that have a wholesale cost to bring into stock - it would be a cashflow negative product to sell if we had to subsidise the premiums on the stock backing unallocated for the duration that they were being held. So it would be possible to create an unallocated product that is sold "at spot" and redeemed below spot, but then someone has to cough up the capital for the premiums on the physical metal that's required to back the product - if we sold an unallocated ounce of silver for $20, but it costs us $23 to bring an ounce of physical silver into stock to back it... there's a shortfall. So we have a premium over spot of $2/oz on silver, $2/g on gold & platinum, and $30/kg on silver, and $30/oz on gold and platinum.
I like to think that Gold Stackers really popularised easy access for unallocated accounts for retail customers in Australia in recent years - when we started offering it few other dealers did. Some even had "unallocated is evil!" type warnings on their websites, but then mysteriously took these warnings down and replaced them with their own unallocated offerings

I probably need to look around again and see who's undercut our spreads, but we have a solid product and many many satisfied customers that just know it works, it's liquid, and it's easy. If you redeem for physical (as our product was really originally intended to be used), then the spreads are moot, as they become a full credit towards your physical redemption in future. We don't use CFDs etc to back the offering which would allow us to offer tighter spreads, it's all backed by physical metal, and in retail sizes too - not piles of illiquid 1000oz bars.
Swapping between unallocated products following a GSR strategy has been popular for a number of our clients. Another way I have seen it used is for personal hedging when turning items in the stack over - a few people will sell items on Silver Stackers or eBay as personal sales, then "back to back" the sale with unallocated at Gold Stackers to retain their overall position in metal - say you wanted to "cash out" some premium, but not reduce your stack - each time you sell a high premium item at a profit, you book the same amount of metal in unallocated (or more if you wanted to reinvest the premium profits as additional metal). Later on you either redeem the unallocated for new products or cash as needed, or hold it as unallocated longer term.